1. Field of the Invention
This invention relates to a process for the nitration of aromatic hydrocarbons and is more particularly concerned with an improved catalytic process for the selective nitration of aromatic hydrocarbons.
2. Description of the Prior Art
The nitration of aromatic hydrocarbons, and, in particular, of benzene and toluene, is normally carried out commercially by reacting the aromatic hydrocarbon with a mixture of nitric and sulfuric acids under controlled temperature; see, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 13, 836-7 and 844-6, 1967. Although many variations of this method have been investigated, it has not yet been possible to devise modifications or improvements which would avoid the use of large volumes of the highly corrosive mixture of nitric and sulfuric acids and also avoid the formation of by-products including the highly undesirable and potentially explosive polynitrated hydrocarbons.
Nitration of hydrocarbons without the use of a mixture of nitric and sulfuric acids has also been described. Illustratively, U.S. Pat. No. 1,325,168 describes passing an oxidizing agent (oxygen) into a mixture of an aromatic hydrocarbon and liquid nitrogen peroxide. U.S. Pat. No. 2,109,873 describes passing a gaseous mixture of an aromatic hydrocarbon and nitrogen dioxide through a bed of silica gel to obtain the mononitro derivative of the aromatic hydrocarbon in low yield. U.S. Pat. No. 2,206,813 describes nitrating aliphatic hydrocarbons in the vapor phase using nitrogen dioxide at temperatures from 300.degree. C to 600.degree. C. U.S. Pat. No. 3,922,315 describes contacting nitrogen dioxide with a liquid phase mixture of an aromatic hydrocarbon and a rhodium catalyst in the presence of hydrogen, U.S. Pat. No. 3,869,253 teaches the nitration of saturated aliphatic hydrocarbons by reaction of the latter with nitrogen peroxide in the presence of oxygen under pressure.
It is an object of the present invention to provide a process which does not require the use of large volumes of a mixture of nitric and sulfuric acids or the use of relatively expensive heavy metal catalysts. It is also an object of the invention to provide a process which is highly selective and gives rise to substantially quantitative yields of the desired nitro compounds with no significant production of by-products, particularly those which are potentially explosive. It is a further object of the invention to provide a process which is readily adaptable to semicontinuous operation. These objects are achieved by the process which is described hereafter.